Getting JSON object with structure but no content??

My json response contains json with structure (eg. "[[[],[]]]") but no actual content.

AJAX code:
function request(url, data, call_back_function) {
var http_request = get_http();

http_request.open("POST", url, call_back_function ? true : false);
http_request.setRequestHeader("content-type", "application/json; charset=utf-8");

if (call_back_function) {
http_request.onreadystatechange = function() {
if (http_request.readyState === 4)
{
call_back_function(http_request.responseText);
} else {
//throw new Error("Web request to " + webmethod + " with data " + data + " failed: " + xhr.responseText);
}
};
}

http_request.send(data)
}


...